In H.264/AVC, which is one of the standard specifications of image coding schemes, individual images (pictures) are partitioned into one or more slices. Each of the slices is classified as one of an I slice (Intra Slice), a P slice (Predictive Slice), and a B slice (Bi-predictive Slice). An I slice is a slice which is independently decoded without referring to another image. A P slice is a slice which is decoded by referring to another single image. A B slice is a slice which is decoded by referring to a plurality of other images. A beginning picture of a sequence including only an I slice is called an IDR (Instantaneous Decoding Refresh) picture. An IDR picture is identified with a value of an NAL (Network Abstraction Layer) unit type. A picture subsequent to an IDR picture in the same sequence is located only after the IDR picture in presentation order without referring to a picture previous to the IDR picture in a decoding order. Accordingly, when random access (not decoding from the beginning of the stream but decoding/reproduction from the middle of a stream) is attempted at a time point of the middle of a video of a certain encoded stream, a video can be appropriately decoded from an IDR picture in the proximity of a designated time point.
In standardization of HEVC (High Efficiency Video Coding) which is the next-generation image coding scheme subsequent to H.264/AVC, a technology for identifying a picture CRA (Clean Random Access) picture with a value of an NAL unit type has been suggested apart from an IDR picture. A CRA picture is a picture including only an I slice in the middle of a sequence. A picture subsequent to a CRA picture in both of a decoding order and a presentation order does not refer to either of a picture previous to the CRA picture in the decoding order or a picture previous to the CRA picture in the presentation order (see Non-Patent Literature 2 below). Accordingly when random access (decoding of a video from the CRA picture) to the CRA picture at a time point of the middle of a video is attempted, a process of decoding a picture subsequent to the CRA picture in the presentation order can be performed without failure.